The present invention relates to a digital X-ray imaging system and method, and more particularly to a digital X-ray imaging system and method suitable for imaging a large field of view such as a chest.
Conventional methods of taking an image of a subject by transmitting therethrough an X-ray are grouped into a method using an X-ray film and a digital radiography (hereinafter abbreviated as "DR") method of taking a digital X-ray image. The DR method is expected to improve a diagnosis performance by using image processing, and can electronically record, store, and search X-ray images. There are known various chest DR methods, including a film digitizing method in which an image taken on an X-ray film is digitized, a storage phosphor digital method in which storage phosphor is used in place of an X-ray film and a latent image is scanned by a laser beam to read it and form a visualized image, a scanography method in which a combination of a one-dimensional X,ray beam and a one-dimensional detector array is used, and an X-ray image intensifier-television camera-method (herein after abbreviated as "X-ray II-TV method) in which uses a combination of an X-ray image intensifier (hereinafter abbreviated as "X-ray II") for converting an X-ray image into an optical image and amplifying it and a television camera for converting the amplified optical image into electric signals.
Of these DR methods, the X-ray II-TV method is also called a real time DR method, has the function of immediately displaying and storing a taken image, and has the shortest time required for taking and processing one image. Therefore, a success or failure of imaging can be judged at once, and this method is suitable for mass screening because of a short time required for one test. It is also suitable for urgent check, and routine as well as accurate diagnosis at hospitals because it takes a short time to obtain the results of imaging and it is possible to provide the functions of sequential imaging, dynamic imaging, fluoroscopy, and the like as well as quick diagnosis. The methods other than the X-ray II-TV method require 30 seconds or longer for imaging and reading image data.
A technique disclosed, for example, in Electromedica Vol. 60(1992), No. 1, pp. 2-5 is known in which the X-ray II-TV method is used to obtain an X-ray image at an imaging target region wider than the field of view of the X-ray detector, by divisionally imaging the region a plurality of times while changing a relative position of the X-ray detector and the subject. According to this technique, a subject lies on a bed in a dorsal position, and the X-ray source and X-ray detector are moved in unison in one direction, for example, in parallel with the longitudinal direction of the subject. An angiogram of lower extremities, for example, is divisionally imaged a plurality of times, and a plurality of obtained images are joined together to display them as a one complete image.
Although the X-ray II-TV method has the above-described superior advantages, conventional DR methods have been accompanied with the problem that one of a field of view and a spatial resolution is inferior to the other methods. It is technically difficult to manufacture an X-ray II which can take an image of a large view field at a ultra high resolution. As an X-ray II having a filed of view as large as about 40 cm * 40 cm necessary for imaging a chest, there is known an X-ray II having a field of view of 47 cm described in Radiology Vol. 171, No. 2 (May, 1989), pp. 297-307). This X-ray II however has a spatial resolution inferior to other methods. Another problem associated with an X-ray II is that the more the position goes apart from the center of the field of view, the more the spatial resolution at the position lowers. Therefore, when lungs are imaged, the central area of the X-ray II having a high resolution images the central region of the mediastinum, whereas the peripheral area of the X-ray II having a lower resolution images the lung field.
With the technique disclosed in Electromedica Vol. 60 (1992), No. 1, pp. 2-5, images (photographs) of a target region divisionally taken a plurality of times are cut and pasted to join them together, and they are not image-processed by a computer. Therefore, image densities at areas around joining lines are discontinuous so that the image quality of a vascular system near the areas around joining lines is poor. In addition, this technique does not take into consideration imaging a plurality of regions of interest at the central area of the X-ray II having a higher resolution.
With the above described divisional imaging method, X-ray beams transmitting through a subject are not parallel beams but diverging beams. Since the X-ray source and X-ray detector are moved relative to the subject, diverging X-ray beams passing through the same position of a subject have different incident angles when imaging the subject a plurality of times. This different incident angle of an X-ray beam generates a positioning error of the subject image in its depth direction, being unable to correctly join a plurality of images.
This problem will be detailed with reference to FIG. 12. An X-ray source 3 and an X-ray detector 16 are moved in unison in parallel to a subject 17 at a dorsal position. As described previously, X-ray beams passing through the subject 17 are diverging beams. Therefore, when the X-ray source 3 is at point A, an X-ray beam 21 incident to the position at point P on the ventral side passes through the position at point Q on the dorsal side, whereas when the X-ray source 3 is at point B, an X-ray beam 22 incident to the position at point P on the ventral side passes through the position at point R on the dorsal side. Since an X-ray image is obtained as an X-ray transmitted image, if the X-ray images taken at positions A and B of the X-ray source 3 are joined by superposing two points P on the ventral side, the joined image is not correct because on the dorsal side, the points Q and R are joined.